Ribonucleic acid (RNA) is essential to the processes which allow translation of the genetic code to form proteins necessary for all cellular functions, both in normal and neoplastic cells. While the genetic code structurally exists as deoxyribonucleic acid (DNA), it is the function of RNA, existing as the subtypes transfer-RNA, messenger-RNA or messenger-like RNA, and ribosomal-RNA, to carry and translate this code to the cellular sites of protein production. In the nucleus, this RNA may further exist as or in association with ribonucleoproteins (RNP). The pathogenesis and regulation of cancer is dependent upon RNA-mediated translation of specific genetic codes, which often reflects mutational events within oncogenes, to produce proteins involved with cell proliferation, regulation, and death. Furthermore, other RNA and their translated proteins, although not necessarily those involved in neoplastic pathogenesis or regulation, may serve to delineate recognizable characteristics of particular neoplasms by either being elevated or inappropriately expressed. Thus, recognition of specific RNA can enable the identification, detection, inference, monitoring, or evaluation of any neoplasm, benign, malignant, or premalignant, in humans and animals. Furthermore, since RNA can be repetitively created from its DNA template, for a given gene within a cell there may be formed a substantially greater number of associated RNA molecules than DNA molecules. Thus, an RNA-based assay should have greater sensitivity, and greater clinical utility, than its respective DNA-based assay. Note that the term RNA denotes ribonucleic acid including fragments of ribonucleic acid consisting of ribonucleic acid sequences.
RNA based nucleic acid amplification assays, including the reverse transcriptase polymerase chain reaction (RT-PCR, also known as reverse transcription polymerase chain reaction or RNA-PCR), branched DNA signal amplification, and self-sustained sequence replication assays, such as isothermal nucleic acid sequence based amplification (NASBA), have proven to be highly sensitive and specific methods for detecting small numbers of RNA molecules. As such, they can be used in direct assays of neoplastic tissue (1-3). Since peripheral blood is readily obtainable from patients with cancer, and metastatic cancer cells are known to circulate in the blood of patients with advanced cancer, several investigators have recently used RT-PCR to detect intracellular RNA extracted from circulating cancer cells (4-7). It must be emphasized that currently investigators apply RT-PCR to detect extracted intracellular RNA from a predominately cellular fraction of blood in order to demonstrate the existence of circulating cancer cells. RT-PCR is applied only to the cellular fraction of blood obtained from cancer patients, i.e., the cell pellet or cells within whole blood. The plasma or serum fraction of blood is usually discarded prior to analysis, but is not examined separately. Since such a cellular fraction approach relies upon the presence of metastatic circulating cancer cells, it is of limited clinical use in patients with early cancers, and is not useful in the detection of non-invasive neoplasms or pre-malignant states.
The invention described by this patent application demonstrates the novel use of that human or animal tumor-derived or tumor-associated RNA found circulating in the plasma or serum fraction of blood, as a means to detect, monitor, or evaluate cancer and premalignant states. This invention is based upon the application of RNA extraction techniques and nucleic acid amplification assays to detect tumor-derived or associated extracellular RNA found circulating in plasma or serum. In contrast to the detection of viral-related RNA in plasma or serum, and the detection of tumor-associated DNA in plasma and serum, the detection of human or mammalian RNA, and particularly tumor-derived or associated RNA, has never been detected specifically within the plasma or serum fraction of blood using nucleic acid amplification methodology, and thus represents a novel and non-obvious use for these RNA extraction methods and nucleic acid amplification assays. Since this invention is not dependent upon the presence of circulating cancer cells, it is clinically applicable to cases of early cancer, noninvasive cancers, and premalignant states, in addition to cases of invasive cancer and advanced cancer. Further, this invention allows the detection of RNA in previously frozen or otherwise stored plasma and serum, thus making plasma and serum banks available for analysis and otherwise increasing general usefulness.
Tumor-derived or tumor-associated RNA that is present in plasma and serum may exist in two forms. The first being extracellular RNA, but the second being extractable intracellular RNA from cells occasionally contaminating the plasma or serum fraction. In practice, it is not necessary to differentiate between intracellular and extracellular in order to detect RNA in plasma or serum using the invention, and this invention can be used for detection of both. The potential uses of tumor-derived or associated extracellular RNA have not been obvious to the scientific community, nor has the application of nucleic acid amplification assays to detect tumor-derived or associated extracellular RNA been obvious. Indeed, the very existence of tumor-derived or associated extracellular RNA has not been obvious to the scientific community, and is generally considered not to exist. It is generally believed that plasma ribonucleases rapidly degrade any extracellular mammalian RNA which might circulate in blood, rendering it nondetectable (8). Komeda et al., for example, specifically added free RNA to whole blood obtained from normal volunteers, but were unable to detect that RNA using PCR (54). However, nucleases appear inhibited in the plasma of cancer patients (9). In addition, extracellular RNA, either complexed to lipids and proteolipids, protein-bound, or within apoptotic bodies, would be protected from ribonucleases. Thus, although still undefined, tumor-derived or associated extracellular RNA may be present in plasma or serum via several mechanisms. Extracellular RNA could be secreted or shed from tumor in the form of lipoprotein (proteo-lipid)-RNA or lipid-RNA complexes, it could be found within circulating apoptotic bodies derived from apoptotic tumor cells, it could be found in proteo-RNA complexes released from viable or dying cells including or in association with ribonucleoproteins, or in association with other proteins such as galectin-3, or RNA could be released from necrotic cells and then circulate bound to proteins normally present in plasma. Additionally it could exist circulating within RNA-DNA complexes including those associated with ribonucleoproteins and other nucleic RNA. Further, RNA may exist within several of these moieties simultaneously. For example, RNA may be found associated with ribonucleoprotein found within proteo-lipid apoptotic bodies. The presence of extracellular RNA in plasma or serum makes their detection by nucleic acid amplification assays feasible.
Several studies in the literature support the existence of tumor-derived or associated extracellular RNA. RNA has been shown to be present on the cell surface of tumor cells, as demonstrated by electrophoresis (10), membrane preparations (11), and P32 release (12). Shedding of phospholipid vesicles from tumor cells is a well described phenomena (13, 14), and similar vesicles have been shown to circulate in the blood of patients with cancer (15). Kamm and Smith used a fluorometric method to quantitate RNA concentrations in the plasma of healthy individuals (55). Rosi and colleagues used high resolution nuclear magnetic resonance (NMR) spectroscopy to demonstrate RNA molecules complexed with lipid vesicles which were shed from a human colon adenocarcinoma cell line (16). Further characterization of these lipid-RNA complexes demonstrated the vesicles additionally contained triglycerides, cholesterol esters, lipids, oligopeptide, and phospholipids (17). Mountford et al. used magnetic resonance spectroscopy to identify a proteolipid in the plasma of a patient with an ovarian neoplasm (18). While further evaluation of the proteolipid using the orcinol method suggested RNA was present, this could not be confirmed using other methods. Wieczorek and associates, using UV spectrometry and hydrolysis by RNases, claimed to have found a specific RNA-proteolipid complex in the serum of cancer patients which was not present in healthy individuals (19-20). The complex had unvarying composition regardless of the type cancer. Wieczorek et al. were further able to detect this specific RNA-proteolipid complex using a phage DNA cloned into E. coli and hybridized to RNA from neoplastic serum, a method distinctly different from the method of this invention. The DNA was then detected by immunoassay (21). However, the RNA found in this complex is described as 10 kilobases, which is so large as to make it questionable whether this truly represents RNA as described. More recently, DNA and RNA-containing nucleoprotein complexes, possibly representing functional nuclear suborganellular elements, were isolated from the nuclei of lymphoma cells (22). It was not shown, however, that these complexes can be shed extracellularly. Other ribonucleoprotein complexes have been associated with c-myc oncogene RNA (56).
While plasma and serum are generally presumed to be cell-free, in the practical sense, particularly under conditions of routine clinical fractionation, plasma and serum may occasionally be contaminated by cells. These contaminating cells are a source of intracellular RNA which is detectable by the methods of the invention. While the level of contaminating cells may be reduced by filters or high speed centrifugation, these methods may also reduce extracellular RNA, particularly larger apoptotic bodies. Clinical utility of the invention is not dependent upon further separating of plasma or serum RNA into its extracellular and intracellular species.
While not related to the claims of this patent, similar analogy likely exists for detection of normal RNA (non-tumor derived or non-tumor associated RNA) in plasma and serum. Subsequent to the filing of the provisional patent application for this patent, the inventor has shown that normal RNA (non-tumor derived RNA) could similarly be detected in the plasma or serum of both healthy volunteers and cancer patients using extraction methods and amplification methods as described by this invention. Qualitative results suggested that amplified product was greater when obtained from cancer patients. Further, use of a 0.5 micron filter prior to amplification reduced, but did not eliminate amplifiable RNA, consistent with extracellular RNA being of variable size, with additional contaminating cells possible.
While the methods of RNA extraction utilized in this invention have been previously used to extract both viral RNA and intracellular RNA, their applicability to extracellular tumor-related or tumor-associated RNA was not obvious. The physical characteristics of the extracellular RNA complexes remain largely unknown, and thus it was not known prior to this invention if the methods of extraction to be described could effectively remove extracellular RNA from their proteo-lipid, apoptotic, vesicular, or protein-bound complexes. This invention describes the applicability of these RNA extraction methods to the extraction of extracellular RNA from plasma or serum, and thus describes a new use for these extraction methods.
In summary, this invention describes a method by which RNA in plasma or serum can be detected and thus utilized for the detection, monitoring, or evaluation of cancer or premalignant conditions. This method utilizes nucleic acid amplification assays to detect human or animal tumor-derived or associated extracellular RNA circulating in plasma or serum. It also enables extraction and amplification of intracellular RNA should cells be present in plasma or serum. The described extraction methods and various nucleic acid amplification assays, including but not limited to RT-PCR, branched DNA signal amplification, transciption-based amplification, amplifiable RNA reporters, boomerang DNA amplification, strand displacement activation, cycling probe technology, isothermal NASBA amplification, and other self-sustained sequence replication assays, have not been used for the detection of tumor-derived or tumor-associated RNA in plasma or serum, reflecting the general scientific bias that mammalian extracellular RNA does not exist circulating in plasma or serum, despite isolated studies to the contrary. Thus, this invention represents both a novel and non-obvious method of detecting, monitoring, and evaluating cancer or premalignant conditions, and a novel and non-obvious application of both RNA extraction methodology and nucleic acid amplification assays. This invention, as described below entails a multi-step procedure applied to plasma or serum which consists of three parts, with the initial step (Part A) involving extraction of tumor-derived or associated RNA from plasma or serum, a second step (Part B) involving application of a nucleic acid amplification assay, in which reverse transcription of RNA to its cDNA may be involved, and a third step (Part C) involving detection of the amplified product. Any nucleic acid amplification assay capable of permitting detection of small numbers of RNA molecules or their corresponding cDNA may be used in Part B. Similarly, various methods of detection of amplified product may be used in Part C, including but not limited to agarose gel electrophoresis, ELISA detection methods, electrochemiluminescence, high performance liquid chromatography, and reverse dot blot methods. Furthermore, Part B and Part C may utilize assays which enable either qualitative or quantitative RNA analysis. Thus, while this invention uses various methods described in the literature, it is the unique application of these methods to the detection of tumor-derived or associated extracellular RNA from plasma or serum that makes this invention novel. This invention provides a simple means for testing blood plasma or serum for tumor-derived or associated RNA, with the result of identifying patients harboring tumor cells. Since this invention enables detection of extracellular RNA, and does not depend upon the presence of circulating cancer cells, it offers a sensitive yet inexpensive screen for both malignancy and pre-malignancy, as well as a way for monitoring cancer and obtaining other prognostically important clinical information.